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About this tool
Extract the taxable value and the GST hidden inside any tax-inclusive amount, split into CGST and SGST or IGST, with optional cess.
A reverse GST calculator recovers the taxable value hidden inside a tax-inclusive price by dividing the gross amount by (1 + rate ÷ 100), then reports the tax as CGST plus SGST for a supply within one state or as IGST for an inter-state supply. It follows section 15 of the CGST Act, 2017, under which GST is charged on the transaction value, and section 170, under which the tax payable is rounded to the nearest rupee. Handy for retailers quoting all-in prices, freelancers converting an agreed fee into an invoice, and anyone checking a restaurant or hotel bill.
Open Reverse GST Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Add an ad valorem compensation cess and the base value still comes out right.
Halves the tax into CGST and SGST for local supplies, or shows the full IGST.
Makes clear that 18% GST is only 15.25% of the gross bill, not 18%.
Divide the total by 1 plus the rate as a decimal. At 18% divide by 1.18, at 12% divide by 1.12 and at 5% divide by 1.05. A Rs 1,180 bill at 18% therefore contains Rs 1,000 of taxable value and Rs 180 of GST.
Because the 18% is charged on the base, not on the total. Tax as a share of the gross amount is rate ÷ (100 + rate), so 18 ÷ 118 = 15.25%. Subtracting 18% from the total instead of dividing by 1.18 understates the base by about 2.7%.
For a supply within one state the tax is split equally — 18% GST becomes 9% CGST plus 9% SGST or UTGST. For a supply between states the whole amount is IGST at the full rate. The split never changes the total tax, only which government receives it.
Yes. Section 170 of the CGST Act requires the tax amount to be rounded to the nearest rupee, with an exact fifty-paise fraction rounded up. The calculator shows both the exact figure and the rounded one so your invoice ties up with your books.
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